The invention relates to a compressor impeller having a number of compressor blades distributed over a circumference thereof.
In turbo-engines, the sealing gap between the rotating blades and the stationary engine housing represents an influencing variable which is of considerable significance for the efficiency of the engine. It is therefore important to keep this sealing gap as small as possible which, because of the high thermal and centrifugal stresses at the blades, proves to be extremely difficult. In order to minimize the sealing gap, it is known to provide turbines with an exterior shroud band, thereby preventing a direct flow around the rotor blade tip. However, exterior shroud bands of this type cannot be produced from one piece because of the strengths of the presently available materials are not sufficient for withstanding the stresses which occur there. For this reason, these shroud bands are composed of individual sections, the number of sections corresponding to the number of blades of one stage, and these sections being molded to the blade tips.
However, this type of a construction is not possible in the compressor field because compressor efficiency, in contrast to that of turbines, is very considerably based on the fact that the blades must be much thinner and must have a slighter curvature as well as sharp leading and trailing blade edges. Therefore, for reasons of strength, it is not possible to mold or mount section-type shroud bands on the outside of compressor blades.
Further, a closed shroud band presents problems because, during operation, the expansions of the compressor disk, the individual blades and the shroud band may mutually interfere with one another and, under certain operating conditions result in tension forces occurring at the shroud band/blade connection which become extremely high. Because of the centrifugal tensions, which are superimposed on one another as well as the thermal stresses which are the result of the high temperatures of up to approximately 600 degrees C. occurring in the operation of modern compressors, it has not been possible so far to produce such one-piece shroud bands.
Thus, it is an object of the present invention to provide a one-piece shroud band for a compressor impeller which can be connected with the compressor impeller without the mutual impairment of the different expansions of the components, and the shroud band is simultaneously centered at the impeller. In addition, a sealing gap between the shroud band and the engine housing is to be achievable which is as narrow as possible.
According to preferred embodiments of the present invention, this object and other objects are achieved in that a ring-shaped shroud band is mounted at the blade tips and each blade tip, slidably in a radial direction, is enclosed by a guide block, the guide blocks being fastened to an interior side of a self-supportingly constructed shroud band.
Advantages of this arrangement include that the shroud band, similar to a centering of spokes, is connected to the compressor impeller. The expansions of the compressor disk and of the rotor blades, as a result of the centrifugal force and the temperatures, lead to a radial shifting of a blade neck in the guide block. At the same time, the self-supportingly constructed shroud band is not affected by these expansions. The expansions of the compressor disk and the rotor blades therefore have no influence on the sealing gap between the shroud band and the engine housing.
This sealing gap is influenced mainly by the expansion of the shroud band and by housing expansions. By a suitable selection of material, the expansion of the shroud band may advantageously be kept at a minimum and, at the same time, the thermally caused expansion of the housing may be coordinated with that of the shroud band, for example, by means of cooling, the selection of material or mechanical pressure devices. The minimizing of the sealing gap can therefore be adapted exclusively to the characteristics of the material of the shroud band, whereas any influences of the other rotating components do not have to be taken into account.
In addition to the improvement of the sealing gap, it is also an advantage that the blade vibrations can be reduced considerably (high damping) and the resistance to strikes by foreign objects can be increased significantly. In addition, an unwinding or untwisting of the blade area during the operation can be prevented. Thus, advantageously, the staggering angle of the blades remains unchanged over the whole rotational-speed range.
In an advantageous further embodiment of the invention, the blade tips are widened for the formation of guide surfaces for interaction with the guide blocks. As a result, the guide surfaces can be precisely manufactured and machined. The guide surfaces are also coated with a coating which inhibits frictional corrosion, thus reducing the danger of fretting. Particularly suitable coatings include those made of tungsten carbide, titanium carbide or titanium nitride.
In an advantageous further embodiment of the invention, filler pieces are provided in intermediate sections between two adjacent guide blocks respectively, these filler pieces being aerodynamically adapted to the blade contour and to the guide block contour. These filler Pieces, which are preferably form-lockingly connected with the shroud band, are made of a material of low density, whereby the overall weight of the arrangement can be lowered considerably without the requirement of accepting a reduction in strength. Titanium foam is particularly suitable for use as the material for the filler pieces since this material has a low density while being resistant to temperature influences and exhibiting a high stiffness.
Another advantageous embodiment of the invention provides that the hollow space in the interior of each guide block between the shroud band and the blade tips is connected with a space downstream of the compressor impeller by way of one or several bores. As a result, it is prevented that the sliding guide between the blade tip and the guide block becomes dirty since air from the area of higher pressure downstream of the compressor blade can penetrate into the hollow space through the bore. The leakage air at the sliding fit therefore takes along dirt particles and prevents a penetrating of the dirt particles between the sliding surfaces.
A preferred embodiment of the invention provides that the material of the shroud band is fiber-reinforced metal, particularly SiC-fiber with a titanium and/or aluminum matrix. Materials of this type have an extraordinarily high strength and, in addition, are so heat-resistant that they can be used in the temperatures of up to approximately 600.degree. C. (in the case of a Ti matrix) which exist in the compressor area. In addition, these materials also have a verY high E-module, whereby the centrifugal-force-induced expansions may be reduced to a minimum. As a result, the sealing gap can be kept approximately constant even in different operating conditions, particularly at different rotational speeds.
In a further advantageous manner, the shroud band has at least one sealing web extending in a circumferential direction so that a sealing gap is situated between the sealing web and the outer housing of the flow duct. This arrangement, which is known in principle, permits a defined adjustment of the sealing gap by the working of small surfaces. In the case of two or several of such sealing webs, aerodynamic phenomena, such as turbulences or the like, contribute to a reduction of the gap flow.
Normally, two sealing webs of this type must be mounted on the shroud band axially behind one another, since this increases the sealing effect with respect to a single sealing web. In this case, the sealing web or webs are made, preferably, of a matrix material, specifically such that by means of a suitable manufacturing process for the shroud band, it is taken into account that the area of the sealing webs to be manufactured remains free of fiber material so that, when the matrix material is filled in, these sealing webs are filled out only by this matrix material.
In order to achieve a sufficient adhesion of sealing webs to the shroud band, the sealing webs, in their cross-section, are provided with a broad base in the area in which the fibers of a fiber-reinforced shroud band end. As a result, a sufficient connection is achieved of the matrix metal between the fibers with the metallic sealing web. In a radially outside direction, i.e., in the direction toward a sealing gap d between the shroud band and housing, the sealing web tapers superproportionally, which is desirable for reasons of centrifugal stress. This also reduces the mass of the sealing ring. A construction of this type has the advantage that repair work can be carried out easily and at reasonable cost by build-up welding of the sealing webs. As an alternative, it is also possible to construct the sealing webs completely of fiber-reinforced material.
As an alternative to the above-mentioned construction, the guide block is widened in a circumferential direction to such an extent that they abut and the filler pieces are no longer required. This type of a construction, which is simpler with respect to manufacturing techniques, is advantageous when fiber-reinforced metals are used which have sufficient strength.
For the manufacturing of a shroud band according to advantageous embodiments of the present invention, the guide blocks are first placed on the blade tips of the rotor blades. Subsequently, the shroud band with the filler pieces is pushed over the compressor impeller. In a further work step, the compressor impeller and the shroud band are centered with respect to one another in a device, the guide blocks are pushed radially toward the outside and, in this position, are connected with the shroud band in a material-locking manner, particularly by soldering, diffusion-connecting or clamping.
The compressor impellers may be constructed according to the blisk (blade and disk) construction method a well as in the form of individual blades. In the latter construction, the blades, as known, may be inserted either into circumferential grooves or into individual grooves. In the case of the construction with individual grooves, the guide blocks may be connected, in a form-locking manner, with the shroud band before the assembly, or may be a component of the shroud band. During the assembly, the rotor blades are first inserted into the guide blocks of the shroud band. Then the whole set of blades is pushed into the grooves of the compressor disk.
Another advantage of preferred embodiments of the invention in connection with the fan blades is that the aerodynamic losses are reduced because the interfering snubbers are absent.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.